Mobile telephones and other portable electronic devices often contain non-volatile memory such as NAND Flash memory. Typically, such applications require memory with a one-time programmable (‘OTP’) memory area. The OTP memory area may be used to store information only once. In a mobile phone application, the OTP memory area is typically used to store the mobile phone International Mobile Equipment Identity (IMEI) number. The IMEI number is used to uniquely identify a particular mobile phone handset. When a mobile phone has been lost or stolen, the IMEI number for that mobile phone is added to a blacklist. Every time the mobile phone attempts to access the cellular network, its IMEI number is checked against the blacklist. If the IMEI number is found on the blacklist, the mobile phone is refused access to the network.
In the past, means were developed for changing the IMEI number on a mobile phone so that a stolen phone could be re-used. In an effort to prevent people from changing the IMEI number, mobile phone manufacturers have increasingly turned to memory manufacturers to supply memories with OTP memory areas. After the OTP memory area has been used to store the IMEI number, and other information, it is not possible to re-program that memory. This effectively prevents a stolen phone from ever being re-used.
The OTP memory area in standard flash memory devices is typically comprised of a page of memory coincident with an array word-line dedicated for OTP purposes. The OTP memory area is also associated with an OTP protection bit. Prior art devices dedicate a single bit of flash memory for storing the OTP protection bit. When this bit is clear, the OTP memory area may be programmed, erased and re-programmed by the user just like any other area of memory in the device. In other prior art devices, the user can only write the OTP, while the erase operation is reserved to the manufacturer. If the OTP protection bit is set, the OTP memory area may no longer be programmed, erased or re-programmed in any fashion. Moreover, once the OTP protection bit is set, the OTP protection bit itself may never be cleared.
In a typical application, such memory devices are shipped from the manufacturer with the OTP memory area and the OTP protection bit erased and thereby enabled for storing data. The mobile phone manufacturer may then program the OTP memory area of the device with, for example, the IMEI number. After final programming of the OTP memory area, the mobile phone manufacturer sets the OTP protection bit which forever locks both the OTP memory area and OTP protection bit and prevents further programming or re-programming.
While such prior art methods of protecting the OTP memory area are generally effective, they are not always reliable. If, for example, the OTP protection bit is cleared, the OTP memory area could again be programmed. Ordinarily, the OTP protection bit may only be erased by the memory device manufacturer. The OTP protection bit may, however, accidentally flip under certain circumstances rendering the OTP memory area vulnerable to erasure and/or reprogramming.
The present inventors have recognized there is therefore a need, for example, for a non-volatile memory device that can more reliably protect the OTP memory area.